The invention relates to a circuit arrangement for evaluation and compensation of the signals from an inductive position sensor, for example as is used in vehicle braking systems.
Pneumatic cylinders are frequently provided in braking systems such as these, with pistons whose piston position can be detected without contact being made, over a wide operating temperature range, generally from −40° C. to +150° C., in all operating states, for example in the presence of oil mist.
In known solutions which are used for these purposes, a plunger-type coil is generally used as a sensor coil, whose coil former has a hole on the longitudinal axis, which a metallic armature composed of ferromagnetic or non-ferromagnetic material enters, therefore varying the inductance of the plunger-type coil. This inductance change can be detected by an electronic evaluation circuit and can be supplied, in the form of a frequency or analog signal, to a microcontroller for further evaluation.
Various types of measurement principles are known for this purpose, and are in general based on measurement of the time constants τ=L/R of the coil. However, it is complex to measure the coil internal resistance, because of the low resistance of the plunger-type coil. Furthermore, if the measurement is carried out as a DC voltage measurement, this results in disadvantages in terms of susceptibility to external magnetic alternating fields and DC voltage shifts within the circuit arrangement that is used, for example as a result of an input voltage shift in an operational amplifier, which result in the position measurement not being sufficiently accurate.
However, an electrical signal which is as accurate as possible must be produced for measurement of the coil internal resistance of the plunger-type coil, providing a good simulation of the instantaneous piston position. At the same time, the output signal from the inductive position sensor or the plunger-type coil must have dynamics which are as good as possible, in order to allow the longitudinal movement of the sensor to be detected with specific sensitivity when position changes occur, and to be insensitive to the external magnetic alternating fields mentioned above, such as those produced by adjacent solenoid valves in the braking system, or by railway lines, scrap processing installations or steel induction furnaces in the vicinity.
The invention is therefore based on the object of providing a circuit arrangement for evaluation and compensation of signals from an inductive position sensor, which is insensitive to external magnetic alternating fields and produces an electrical signal with better accuracy and good dynamics.
According to the invention, this object is achieved by the features of patent claim 1, and alternatively by the features of patent claim 2.
Advantageous developments of the invention are the subject matter of the attached dependent claims.
The object of the invention is therefore solved by a circuit arrangement for evaluation and compensation of the signals from an inductive position sensor, characterized by:
a first operational amplifier to whose inputs a reference voltage is supplied;
a second operational amplifier to a first of whose inputs the output signal from the first operational amplifier is supplied and to a second of whose inputs a feed-back signal is supplied for closed-loop amplitude control; and
a coil having a coil inductance and a coil resistance for position measurement, which coil is connected in parallel with the output of the second operational amplifier and the first input of the second operational amplifier and, in conjunction with a capacitance, which is connected in series with the coil inductance and the coil resistance, forms an RLC series resonant circuit.
This in its own right results in the RLC series resonant circuit having a resonant frequency of high accuracy and with good dynamics, and which is insensitive to external magnetic alternating fields. If this resonant frequency still includes inaccuracies which are intolerable (for example induction changes caused by changes in the permeability μr of the material over the temperature in the magnetic circuit), by an alternative circuit arrangement for evaluation and compensation of the signals from an inductive position sensor the accuracy of the resonant frequency can be further increased.
The alternative circuit arrangement for evaluation and compensation of the signals from an inductive position sensor is characterized by:
a first operational amplifier to a first of whose inputs a first reference voltage is supplied for a frequency measurement or a second reference voltage is supplied for a compensation measurement, and to a second of whose inputs the output signal from the first operational amplifier or a compensation signal is supplied;
a second operational amplifier, to a first of whose inputs the output signal from the first operational amplifier is supplied and to a second of whose inputs a feed-back signal is supplied for closed-loop amplitude control;
a first coil having a first coil inductance and a first coil resistance for a position measurement, which coil is connected in parallel with the output of the second operational amplifier and the first input of the second operational amplifier, and, at a first of its ends and in conjunction with a capacitance which is connected in series with the coil inductance and the coil resistance, forms an RLC series resonant circuit; and
a second coil having a second coil inductance and a second coil resistance for temperature and/or disturbance voltage compensation, which second coil is connected at a first of the ends of its coil winding to a second end of the coil winding of the first coil, and can be connected at a second of the ends of its coil winding to the second input of the first operational amplifier, the accuracy of the resonant frequency be further increased.
In a further preferred embodiment of the circuit arrangement, the first coil is a plunger-type coil with a plunger-type armature, and the RLC series resonant circuit is an active resonant circuit, whose output frequency is independent of the series resistances in the resonant circuit and is proportional to the position of the plunger-type armature in the first coil, and in which the position measurement is carried out using a resonance method based on AC voltage, such that the resonant frequency of the position measurement is significantly higher than an externally induced disturbance frequency. This advantageously results in the circuit arrangement having a highly stable response to external disturbances.
Furthermore, the plunger-type armature is preferably composed of a ferromagnetic or non-ferromagnetic material.
If the dielectric of the capacitance is composed of a temperature-stable material, and if the temperature-stable material is advantageously, for example, a C0G or NP0 ceramic, the temperature response of the capacitance and therefore of the output frequency of the resonant circuit can be minimized and stabilized.
In the circuit arrangement whose resonant frequency is more accurate, a first switch is preferably provided for application of the first reference voltage to the first input of the first operational amplifier, a second switch is preferably provided for application of the second reference voltage to the first input of the first operational amplifier, and a third switch is preferably provided for amplification of a third reference voltage, and therefore of a constant difference voltage across the first coil, between the capacitance and the coil resistance of the first coil. This makes it possible to switch in a simple manner between position measurement and the additional temperature compensation since the circuit arrangement can advantageously be switched, by means of the first to third switches, between a position measurement and temperature compensation, and/or compensation for magnetic disturbance fields on the first coil.
Particularly preferably, in order to provide compensation for the measurement coil, the coil windings on the first and on the second coil form a bifilar winding with identical coil inductances, identical coil resistances, and with the coil windings connected in opposite senses, thus allowing simple detection of the temperature by evaluation of the plunger-type coil internal resistance by means of a suitable circuit, using the coil current or a voltage applied across the coil.
The output signals that are produced are therefore a digital frequency or position signal at a first output, which signal is proportional to the insertion depth of the plunger-type armature in the first coil, and/or an analog temperature signal at a second output, which signal is proportional to the temperature of the plunger-type coil, in which case this can advantageously be achieved as a function of the required characteristics and accuracies by mutually separate circuit parts or alternatively by combined circuit parts, by means of a first circuit part for the position measurement, which produces the digital frequency or position signal at the first output, and by means of a second circuit part for the resistance measurement, which produces the analog temperature signal at the second output.
On the one hand, the invention is therefore based on the idea of providing a first circuit part for the position or frequency measurement, in which a measurement coil which acts in an RLC series resonant circuit is used to measure and produce a suitable output signal, and on the other hand additionally on the use of a second circuit part, which uses a resistance measurement to allow temperature compensation and compensation for inaccuracies, induced by external magnetic disturbance fields, in the temperature compensation of the measurement coil, and which therefore allows even better measurement accuracy. Both of the abovementioned circuit parts are largely insensitive to temperature fluctuations and interference from external magnetic fields, therefore in particular reducing the sensitivity of the circuit to magnetic disturbance fields.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description when considered in conjunction with the accompanying drawings.